Method of piercing in seamless tube manufacturing

Abstract

The invention relates to a method of rotary piercing in seamless tube manufacturing under a rotary piercing process, such as Mannesmann mandrel mill process. Rotary piercing operation is carried out by employing a rotary piercing mill having a pair of cone-shaped main rolls adapted to cooperate with a plug for rotary piercing and disc rolls disposed in opposed relation between the pair of main rolls and adapted to press hollow shell on the surface thereof. In this method of rotary piercing, the main rolls have feed and cross angles designed to meet certain conditions individually and in combination. The method makes it possible to carry out rotary piercing of less hot workable and/or extremely hard-to-work materials, that is, high-alloy steel billets, without surface torsional deformation and circumferential shear deformation. Thus, it is possible to manufacture high quality tubes of high-alloy steels free from outside seams and inside bore defects.

What is claimed is: 
     
       1. A method of rotary piercing in seamless tube manufacturing wherein a heated billet is fed into the roll gap between opposed rolls and subjected, while being moved forward in rotation on its axis and in the axial direction, to center rotary piercing by a plug disposed between the rolls until it is turned into a hollow shell, comprising employing a rotary piercing mill having main rolls disposed in horizontally or vertically opposed relation, with a billet/hollow-shell pass line between, and rotating disc rolls disposed in vertically or horizontally opposed relation between the main rolls, with the pass line between, said main rolls being so arranged as to have a feed angle β and cross angle γ meeting the following conditions:   3°<β<25°       3°<γ<25°       15°<β+γ<45°,     said rotating disc rolls being pressed against the billet and hollow-shell during the rotary piercing operation to produce a hollow shell with substantially reduced circumferential shear deformation and internal bore defects.   
     
     
       2. A method of rotary piercing as set forth in claim 1, wherein said disc rolls are rotated by a drive motor separate from that for the main rolls so as to assist in billet engagemet with the main rolls. 
     
     
       3. A method of rotary piercing as set forth in claim 1 or 2, wherein the rotation speed of said disc rolls are determined in relation to sin β. 
     
     
       4. A method of rotary piercing as set forth in claim 1, wherein the outside diameter of said disc rolls each is greater than the maximal outside diameter of each of the main rolls. 
     
     
       5. A method according to claim 1 wherein said main rolls are generally conical. 
     
     
       6. Apparatus for rotary piercing in seamless tube manufacturing comprising: a pair of opposed main rolls defining a roll gap into which a heated billet is fed while being moved forward in rotation on its axis and in the axial direction, said main rolls disposed in horizontally or vertically opposed relation, with a billet/hollow-shell pass line between,   a plug disposed between the rolls to pierce the heated billet until it is turned into a hollow shell; and   rotatable disc rolls disposed in vertically or horizontally opposed relation between the main rolls, with the pass line between, said main rolls being so arranged as to have a feed angle β and cross angle γ meeting the following conditions:   3°<β<25°       3°<γ<25°       15°<β+γ<45°,        said disc rolls being pressed against the billet and hollow-shell during the rotary piercing operation to produce a hollow shell with substantially reduced circumferential shear deformation and internal bore defects.   
     
     
       7. Apparatus as set forth in claim 6, further comprising a drive motor separate from the main rolls for rotating said disc rolls and to assist in billet engagement with the main rolls. 
     
     
       8. Apparatus as set forth in claim 7 wherein the speed of rotation of said disc rolls is proportioned to sin B. 
     
     
       9. Apparatus as set forth in claim 8 wherein said drive motor for said disc rolls rotates said disc rolls at a speed which is proportioned to sin β. 
     
     
       10. Apparatus as in claim 6 wherein the outside diameter of said disc rolls is greater than the maximum diameter of said main rolls. 
     
     
       11. Apparatus as in claim 6 wherein said main rolls are generally conical.